This invention relates to a traveling screw which is commonly used in the wood yard of a paper mill to extract wood chips from a silo or outdoor pile for conveyance to the pulping operation. More particularly, this invention relates to a unique manner of mounting the ends of the screw to accommodate a predetermined amount of axial movement and lateral skewing in the horizontal plane in which the screw travels in order to alleviate damaging stress in the screw and its mountings. It can also accommodate a small amount of vertical skewing.
Typical prior art traveling screw arrangements are shown in U.S. Pat. No. 3,648,860 and a C. J. Wennberg Co. advertisement entitled "Bank=Money" from the Nov. 15, 1978 issue of the Paper Trade Journal.
There are several ways of removing chips from a wood pile, such as loading a traveling conveyor belt with a bulldozer operating on a pile of chips stored outside. However, bulldozers require a very heavy capital investment as well as the labor cost of an operator. In addition, bulldozers tend to remove the chips from the top of a pile while the chips at the bottom remain for an overly long period of time which sometimes results in loss of usable fiber by biological-chemical reactions. In addition, the bulldozer tends to crush and ruin a portion of the chips in the normal course of its operation. By comparison, traveling screws remove chips from the bottom of a silo or outside stack so that the silo or stack can be replenished with fresh chips simultaneously with the removal of previously stored chips. This first-in-first-out procedure promotes uniformity in chip storage time and reduces loss due to biological/chemical reactions brought on by degeneration when chips have been stored for long periods of time, particularly outside.
However, in prior art types of traveling screws, there are often breakdowns in the traversing carriages, screw shaft or support mountings and the rack and pinion teeth by which the carriage is moved. Also, variations in the density, mechanical structure or degree of compaction of the material impose different forces and loads on the screw at different places along its length as it travels in its traversing movement. All of these factors can and do cause the screw to skew and move axially relative to its end mountings which are not capable of accommodating such movement. Eventually, there is a breakdown in either the drive mechanism or end support brackets and the entire screw halts. Ironically, these breakdowns are caused in part by the theoretically efficient design and operating characteristics of the mechanical system for powering the traversing movement of the screw. Such prior art screws are powered by one motor which operates a rack and pinion at one end and rotates a drive shaft extending through the cylindrical core of the screw to operate a similar rack and pinion at the far end. Each end of the screw is thus moved at the same speed which provides the uniform traverse movement under ideal conditions.
However, the tracks on which the carriage supporting the ends of the screw ride are not perfectly straight. Further, material often gets on the tracks and retards transverse movement of one end of the screw relative to the other end. Variations in the density and compaction of the material cause corresponding variations on the lateral forces acting against the screw as it travels transversely which causes one end of the screw to try to move relative to the other end. Thus, when, for any reason, one end of the screw doesn't move at the same speed as the other end, severe stresses are raised both within the screw cylinder, the internal driving shaft and the brackets in which the end bearings are mounted. Therefore, even though the prior art type of screw design should be efficient because it is powered by a single motor, and should move transversely at the same speed at both ends because of the shaft connecting the racks and pinions on either end, the screw in fact doesn't operate well in continuous service under a wide range of operating conditions.
These problems require more maintenance, supervision and downtime, all of which is costly. This is particularly true with regard to the downtime when repairing the flights on the screw or replacing the entire screw because the drive shaft extending through the screw core requires additional couplings at either end of the screw as well as some sort of support arrangement, such as a bearing, to support the shaft for rotation relative to the screw core. Further, sand is often present, particularly in hogged bark chips, and this causes the screw flights to wear rapidly so that the screw has to be replaced or repaired. The downtime to replace or repair the screw is very costly regardless of the cause.